A somatic cell that has just completed the S phase of its cell cycle, as compared to gamete of the same species, has

twice the number of chromosomes and four times the amount of DNA.

twice the number of chromosomes and twice the amount of DNA.

same number of chromosomes but twice the amount of DNA.

four times the number of chromosomes and twice the amount of DNA.

In 'S' phase of the cell cycle

Amount of DNA doubles in each cell

Amount of DNA remains same in each cell

Amount of DNA is reduced to half in each cell

S Phase

S நிலை

NCERT Class 11 · Cell Cycle and Cell Division · 6 NEET questions

Most students confuse DNA replication with chromosome duplication. In S phase, DNA content doubles, but the chromosome number remains constant. If you miss this distinction, you lose four marks.

The idea

S phase is the synthesis phase of the cell cycle. During this period, the amount of DNA per cell doubles. If the initial amount of DNA is denoted as 2C, it increases to 4C. However, the chromosome number remains 2n. Centriole duplication also initiates in the cytoplasm during this phase.

Why it matters

It ensures that each daughter cell receives an identical and complete set of genetic information.

NEET lens

NEET focuses on the DNA:Chromosome ratio. Remember: DNA content doubles (2C to 4C), but chromosome number remains the same (2n). Do not confuse this with Anaphase where chromosomes split. Memorize that centriole duplication occurs in the cytoplasm, not the nucleus.

Clinical significance

Errors during S phase lead to genomic instability and mutations. In oncology, chemotherapy drugs like antimetabolites specifically target the S phase to arrest rapidly dividing cancer cells. Failure in DNA replication fidelity is the primary driver of carcinogenesis.

Introduction

S phase (Synthesis phase) is the stage of interphase during which DNA replication occurs, doubling the cell's genetic content from 2C to 4C in preparation for cell division.

Core concept

During S phase: each chromosome is replicated semiconservatively — each new double helix has one original strand + one new strand. Replication begins at multiple origins of replication (ori) simultaneously. DNA content doubles (2C → 4C) but chromosome number stays the same (each chromosome now consists of two sister chromatids joined at centromere). Histones are also synthesized during S phase.

Go deeper

Replication timing: not all chromosomes replicate at the same time in S phase. Euchromatin (transcriptionally active) replicates early in S phase. Heterochromatin (condensed, inactive) replicates late. The inactive X chromosome (Barr body) replicates very late in S phase.

From Class 10

Before cell division, the cell must copy all its DNA so each daughter cell gets a complete set. This copying happens during S phase.